WINCKWORTH : SCHEME FOR BRITISH MARINE CENSUS. I53 



The main object is to get a system which shall really help to repre- 

 sent the British distribution of marine species, and stimulate interest 

 in their study. 



Admiralty Chart No. 2 — British Isles — is a convenient chart to 

 work on, giving the whole "British marine area as far as 15° West, on 

 the average scale of 2|- inches to a degree. 



For land and freshwater moUusca, counties and sub-divided coun- 

 ties have been taken as the census unit, but for marine mollusca I 

 believe a far larger area would in most cases be more satisfactory. 

 Even for coastal areas, the county borders do not as a rule divide up 

 the coast suitably; for instance, both sides of an estuary should be 

 in the same area ; nor should important centres of biological research, 

 as Liverpool or Plymouth, be on the edge of an area. For littoral 

 and shallow water species, perhaps, these areas may be conveniently 

 subdivided, but too great a subdivision is here also undesirable, so 

 that the coast-line may include suitable habitats for local species ; 

 while further search is likely to level up the records of adjacent dis- 

 tricts. Thus in East Sussex I have found several species, notably 

 Acanthochites discreparis and Lepton squamosum, previously recorded 

 only from further west. On the other hand smaller areas give 

 greater value to records from near the border, when tabulated. 



Mr. Tomlin, who saw this paper some time ago, strongly recom- 

 mended separate littoral areas (to include shallow water dredgings), 

 since as a rule the fauna is so distinct and recognizable, while littoral 

 areas would conveniently be much smaller than submarine areas ; 

 and it would be a good plan to go round our coasts and designate a 

 number of places as centres of littoral districts. The definition of 

 littoral is difficult ; I suggest hesitatingly that waters within five miles 

 of the shore and less than twenty-five fathoms in depth may be taken 

 as an initial definition ; but drawing a sharp theoretical line where 

 nature moves gradually is never easy. Perhaps it would be well 

 not to have too cast-iron a limit at first, but to accommodate it to 

 what we already know. 



A good boundary line should be ' neutral ' for the majority of 

 species, i.e., species found at the boundary should be well distributed 

 into either adjacent area, while a disappearing species should be 

 found only on one side of the boundary. Thus I have taken the 

 southern limit of Yorkshire at 54° N., well south of the chalk cliffs 

 on the shore and of the hard ground further out, and well north of 

 the Humber. The now typical east coast species Crepidula forriicata 

 and Peirlcola pholadifonnis (both recent introductions from America) 

 are sometimes found on the Yorkshire side of the Humber, but never 

 I think as far north as 54S. At first I used the fifty fathom line as 



